I don't know all that much about the history, I've just spent some time on this specific topic.
I can't speculate about motives (at least not usefully) of historians, but I assume you're talking about Tacitus in this case. His account records that there were Christians persecuted for their beliefs, which is well established, and he dates it to late Nero, Tacitus writes only about 50 years after the persecution. That is pretty good evidence that people in Rome believed a thing that supposedly came out of Judea, and that they were persecuted for it. Where it's not helpful is establishing any link between events in Judea and this later belief in Rome, because it merely restates beliefs of Christians at the time (yr ~120), referring to a time that happened a generation after Jesus. But I do agree it's the best piece of writing we have and the best place to make an argument.